Several investigations have pointed to a possible need for distinguishing between "brightness" and "sensitivity" as they are defined, respectively, by reference to brightness-matching (estimation) vs. increment-threshold procedures. This project is designed to provide further evidence relating to whether brightness and sensitivity are mediated by common mechanisms through a comparison of the spatial Broca-Sulzer effect and Sensitization effect (Westheimer, 1967) at brief exposure durations. Each of these effects denotes a trend in the intensity-area relationship (i.e., negative coefficients of spatial summation) that is not predictable from previous threshold-level studies. Specifically, the former effect refers to the finding that, for stimuli which are fixated foveally and viewed at suprathreshold levels, a disc-shaped stimulus subtending 2'--3' appears brighter than a larger stimulus of the same retinal illuminance. The sensitization effect refers to the fact that increasing the diameter of an adapting-field increases rather than decreases the increment threshold sensitivity of the visual system. Westheimer previously reported a marked attenuation of the photopic sensitization effect with an adapting field duration of 10 msec. In contrast, the present research has revealed comparatively little attenuation of the spatial Broca-Sulzer effect at durations as brief as 10 msec. Consequently, if the spatial Broca-Sulzer effect is interpreted as reflecting the operation of an inhibitory mechanism acting to depress the brightness of the larger-diameter stimuli, then it seems likely that the type of inhibition is more similar to that producing the Mach band phenomenon than to that thought responsible for the sensitization effect. Consistent with this viewpoint are recent results indicating that equally-bright stimuli of different size do not produce equivalent changes in increment threshold sensitivity.